The present invention relates to a process for the preparation of a granular NPK (nitrogen-phosphorus-potassium) fertilizer, in which a solid potassium salt, such as potassium chloride, is mixed with an ammonium nitrate-containing liquid phase that also contains ammonium phosphate, or to which ammonium phosphate is added as the next step, and the mixture thus obtained is granulated.
Forming the desired NPK granules is relatively convenient, however, the problem attending such a process is the disintegration of the granules during storage, in particular if the temperature at which the product is stored is above 35.degree. C. Disintegration as used herein denotes the phenomenon that during storage continuing chemical reactions and the attendant formation of new crystals cause the granules to exhibit a strong decrease in breaking strength, to start cracking at the surface, and to show serious efflorescence of salts at the granule surface, which results in annoying dust formation and an increase of fine particles. These conditions lead to diminished product value, increased handling costs and unwarrented worker exposure.
The efflorescence in turn gives rise to coalescence of the individual grains causing the so-called caking of the fertilizer. While it is possible to suppress these phenomena either by heating the potassium salt before the addition, or by heating the entire mixture of N compound, P compound and K compound and thoroughly mixing it before granulation, still in all these measures require a rather considerable consumption of energy, and further comparatively large processing equipment is needed owing to the longer mixing time.
The object of the present invention is to provide a simple, economical and direct process for the preparation of stable granular NPK fertilizers in which the above-mentioned disadvantages are substantially or completely avoided.